1. Field of the Invention
The present invention relates to a process for producing a foamed polymer. More particularly, the present invention relates to a process for producing a foamed isocyanate-based (e.g. polyurethane, polyurea, polyisocyanurate, etc.) polymer.
2. Description of the Prior Art
Isocyanate-based polymers are known in the art. Generally, those of skill in the art understand isocyanate-based polymers to be polyurethanes, polyureas, polyisocyanurates and mixtures thereof.
It is also known in the an to produce foamed isocyanate-based polymers. Indeed, one of the advantages of isocyanate-based polymers compared to other polymer systems is that polymerization and foaming can occur in situ. This results in the ability to mold the polymer while it is forming and expanding.
One of the conventional ways to produce a polyurethane foam is known as the "one-shot" technique. In this technique, the isocyanate, a suitable polyol, a catalyst, water (which acts as a reactive blowing agent and can optionally be supplemented with one or more blowing agents) and other additives are mixed together at once using, for example, an impingement (i.e. high pressure) mixer. Generally, if one were to produce a polyurea, the polyol would be replaced with a suitable polyamine. A polyisocyanurate may result from cyclotrimerization of the isocyanate component. Urethane modified polyureas or polyisocyanurates are known in the art. In either scenario, the reactants would be intimately mixed very quickly using a suitable mixer.
Another technique for producing foamed isocyanate-based polymers is known as the "prepolymer" technique. In this technique, a prepolymer is produced by reacting polyol and isocyanate (in the case of a polyurethane) in an inert atmosphere to form a liquid polymer terminated with isocyanate groups. To produce the foamed polymer, the prepolymer is thoroughly mixed with a lower molecular weight polyol (in the case of producing a polyurethane) or a polyamine (in the case of producing a modified polyurea) in the presence of a curing agent and other additives, as needed.
Regardless of the technique used, it is known in the an to include a filler material in the reaction mixture. Conventionally, filler materials have been introduced into foamed polymers by loading the filler material into one or both of the liquid isocyanate and the liquid active hydrogen-containing compound (i.e. the polyol in the case of polyurethane, the polyamine in the case of polyurea, etc.).
The nature and relative amounts of filler materials used in the reaction mixture can vary, to a certain extent, depending on the desired physical properties of the foamed polymer product, and limitations imposed by mixing techniques, the stability of the system and equipment imposed limitations (e.g. due to the particle size of the filler material being incompatible with narrow passages, orifices and the like of the equipment).
Typically, when it is desired to load the foamed polymer with a filler material, there are limitations on the process resulting from the increase in the viscosity of the reaction mixture as polymerization and foaming proceed. Additional limitations result from the difficulties encountered in achieving substantially complete wetting-out of filler materials in the case where all ingredients of the reaction mixture (including the filler material) are mixed in one step in a suitable mix head (i.e. the "one-shot" technique).
A particular difficulty is encountered in the situation where the nature and surface structure of the filler material renders it selectively compatible with some but not all of the ingredients in the reaction mixture. The result of this is that, regardless of whether adequate mixing of ingredients is achieved, there is an imbalance in the physical allocation of the filler material in the foamed polymer product with the filler material essentially agglomerating. This results in non-uniform physical properties in the foamed product. A secondary effect is the relative (and localized) withdrawal of one or more components of the homogeneous, liquid reactant system which may cause catastrophic foam property alterations.
When a filler material is added to any reaction mixture used to produce a foamed isocyanate-based polymer, it is desirable during the process to achieve both (i) uniform distribution of the filler material throughout the polymer matrix, and (ii) intimate contact (both chemical and physical) between the filler material and the isocyanate and active hydrogen-containing compound. The reason for this is that the desired product is a polymer which is cellular in nature and the cells should not be significantly disrupted by the presence of the filler material. The result of not achieving both (i) and (ii) above can cause uncontrolled physical property variations in the resulting foamed product due to an uneven distribution of the filler material. This is particularly a problem in the case when the individual particles of filler material are not separated from each other and the resulting foamed product contains lumps of either "dry" or "wetted" and agglomerated filler material particles.
Attempts have been made to overcome these problems by addition of the filler material to the reaction mixture in specially designed low pressure mixing heads. These mixing heads essentially endeavor to achieve both (i) and (ii) in a single step. While these mixing heads provide for adequate mixing of the filler material and the reaction mixture, they may necessitate alteration of the chemical composition in order to produce an acceptable foam with a high loading of filler. Therefore, it is not typically possible to obtain high loadings of filler material due to the fact that the mixing heads do not provide uniform distribution of the filler material simultaneously with the required intimate mixing (at the molecular level) of the main chemical reactants. The reason for this is that, as the polycondensation reaction proceeds, the viscosity of the reaction mixture increases resulting in a reduction in the ability to achieve (i) and (ii) above and the presence of substantial amounts of filler may cause energy loss of the reacting system (i.e. a heat sink effect). Practically, this translates into an inability to achieve filler loading levels of greater than about 17-30% by weight of the polymer reaction mixture without adversely affecting the physical properties of the final foamed polymer product. Furthermore, if the efficiency of mixing the reactants and filler material is inadequate, a separation effect of the filler material (by particle size) is likely to occur resulting in an uneven particle size in the cross-section of the foam mass.
In copending U.S. patent applications Ser. Nos. 08/007,991 (filed Jan. 22, 1993) and 08/074,966 (filed Jun. 11, 1993), the contents of each of which are incorporated herein by reference, there is disclosed a foamed isocyanate-based polymer containing a filler material, the filler material being present at a level greater than 30% by weight of the polymer and having a specific gravity of less than about 2.0 with proviso that the filler material is not melamine. A process for producing the foamed isocyanate-based polymer is also disclosed. The process comprises the steps of: providing a first mixture comprising water, a catalyst and a filler material, the filler material being wetted by the water and the catalyst; providing a second mixture comprising an isocyanate and an active hydrogen-containing compound; mixing the first mixture and the second mixture to provide a reaction mixture; and allowing the reaction mixture to expand to produce the foamed isocyanate-based polymer. An important aspect of the process is the provision of a second mixture comprising the isocyanate and the active hydrogen-containing compound substantially free of any catalysts or water so that gelling thereof is retarded and no foaming can occur. This results in a second mixture which is a liquid having a viscosity sufficiently low that it may be handled relatively easily using conventional equipment.
While the process described in the '991 and '966 applications is very useful and versatile, there can exist difficulties, depending on factors such as the nature of the filler material and the like, associated with adapting the process into a commercial operation. A principal difficulty is the requirement to have very accurate metering of the filler material. This is required since the filler material is acting as a carrier for the catalyst and water which serve to initiate polymerization and foaming, respectively, of the reaction system. As is known in the art of isocyanate-based foams, the quality of the foam is dictated, at least in part, by ensuring that a prescribed amount of catalyst and water is used in the reaction system. In producing a non-filled isocyanate-based foam, these components would conventionally be dispersed in one of the liquid reactants thereby facilitating metering thereof. By removing the catalyst and water from the liquid reactants and using them to coat or size the filler material, metering of the catalyst and water becomes dependent on accurately metering a solids stream comprising the filler material. Generally, accurate metering (as required by the specific system being metered) of a solids stream is more difficult to achieve than accurate metering of a liquids stream.
In light of these difficulties, it would be advantageous to have a process for producing a foamed isocyanate-based polymer which is relatively simple, can utilize a large variety of filler materials and allows for the introduction of substantially large amounts of filler materials. It would be especially advantageous if such a process (i) could be adapted to utilize filler materials based on recycled isocyanate-based foams or elastomers or other post-consumer and post-user products (e.g. tires), and (ii) was not limited by the particle size of the filler material. Further, it would be especially advantageous if the process was less dependent on the ability to accurately meter the filler material. Still further, it would be advantageous if the process could be relatively easily switched from making a filled faom to making an untilled foam, and vice versa.